|  | 1.1 | The OSI Reference Model and the Problems it Solves
| 1.1.3 | Peer-to-peer communication
The OSI reference model describes how information makes its way from application programs on different computers through a network medium. As the information to be sent descends through the layers of a given system, it looks less and less like a human language and more and more like the ones and zeros that a computer understands. Each layer uses its own layer protocol to communicate with its peer layer in the other system. Each layer's protocol exchanges information, called protocol data units (PDUs), between peer layers. The figure shows an example of OSI-type communication. Host A has information to send to Host B. The application program in Host A communicates with Host A's application layer, which communicates with Host A's presentation layer, which communicates with Host A's session layer, and so on, until Host A's physical layer is reached. The physical layer puts information on (and takes information off) the physical network medium. After the information traverses the physical network medium and is picked up by Host B, it ascends through Host B's layers in reverse order (first the physical layer, then the data link layer, and so on) until it finally reaches Host B's application layer. Although each Host A layer communicates with its adjacent layers, each layer in a host has a primary task it must perform. The primary task of each layer is to communicate with its peer layer in Host B. That is, the task of Layer 1 in Host A is to communicate with Layer 1 in Host B; Layer 2 in Host A communicates with Layer 2 in Host B, and so on. The OSI reference model's layering prohibits direct communication between peer layers in different hosts. Each layer in Host A must therefore rely on services provided by adjacent Host A layers to help achieve communication with its Host B peer. Assume that Layer 4 in Host A must communicate with Layer 4 in Host B. To do this, Layer 4 in Host A must use the services of Layer 3 in Host A. Layer 4 is said to be the service user, and Layer 3 is the service provider. Layer 3 services are provided to Layer 4 at a service access point (SAP), which is a location at which Layer 4 can request Layer 3 services. Thus, the TCP segments become part of the network layer packets (also called datagrams) exchanged between IP peers. In turn, the IP packets must become part of the data link frames exchanged between directly connected devices. Ultimately, these frames must become bits as the data is finally transmitted by the physical-layer protocol using hardware.
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